U.S. Pat. No. 2,833,816 disclosed the unique catalysis provided by the combination of a source of bromine with one or more metal oxidation catalyst for oxidizing with a source of molecular oxygen gas (e.g., air) all the alkyl-substitutents of alkyl-substituted aromatic hydrocarbons to an aromatic di-, tri- or higher polycarboxylic acid (e.g. oxidation of a xylene to a phthalic acid) under liquid phase conditions. Since such disclosure, there has been an interest shown from time to time for the use of said catalytic liquid phase oxidation for the conversion of o-xylene to the anhydride of o-phthalic acid.
For example, U.S. Pat. No. 3,402,184 disclosed a continuous process for oxidizing o-xylene in an acetic acid solution containing ions of cobalt, manganese and bromine with air in three steps. Such o-xylene oxidation results in a solution of phthalic anhydride ("PAN") in the acetic acid. Said solution of PAN is combined with water and heated to convert the dissolved PAN to insoluble o-phthalic acid product.
British Patent Specification No. 856,245 published in 1960 discloses that a single step, neat (no extraneous solvent) oxidation of o-xylene does not go beyond 70 mole percent conversion of the xylene to phthalic anhydride but a combination of fresh xylene and the 30 percent unoxidized cannot be oxidized.
U.S. Pat. No. 3,920,735 discloses that the neat, single step oxidation of o-xylene with air in the presence of cobalt, manganese, zirconium and bromine produces a 60 mole percent yield of phthalic acid together with 0.97 mole percent yield of o-toluic acid, 2.95 mole percent yield of phthalide and 1.88 mole percent yield of 2-carboxybenzaldehyde. Such a PAN product is quite impure and contains a relatively large amount of difficulty removable phthalide.
Lastly, copending U.S. Pat. application Ser. No. 867,050, filed Jan. 5, 1978 describes an improved neat oxidation of o-xylene in the liquid phase to 85 to 92 mole percent yields of o-phthalic acid. The key to such substantially higher yield is to maintain the o-xylene oxidation product as the free dicarboxylic acid; that is, o-phthalic acid, and avoid conditions which permit such acid to dehydrate to its intramolecular anhydride, PAN. Such conditions are the conduct of the neat oxidation of liquid o-xylene with air at a temperature in the range of from 150.degree. C. up to 235.degree. C., at a gauge pressure of from 17 up to at least 28 kg/cm.sup.2 and in the presence of from 2 up to 7 weight percent water in the reaction mixture. Satisfactory rates of oxidation are, under those conditions, obtained in the presence of catalysis provided on the basis of one gram mole of o-xylene by from 0.3 up to 10 milligram atoms of cobalt, from 0.15 up to 20 milligram atoms of manganese and from 0.225 to less than 60 milligram atoms of bromine. That is, the ratio of gram atoms of bromine per gram atom total of cobalt and manganese is at least 0.5:1 but is less than 2:1. Also, when the cobalt concentration is less than 0.75 milligram atom per gram mole of o-xylene than the difference between the actual cobalt concentration and the 0.75 milligram atom per gram mole of o-xylene can be supplied by zirconium on a gram atom for gram atom basis even though zirconium is not a polyvalent transition metal.
Said higher conversion and yield performing process is illustrated by and is limited to the use of batchwise or semi-continuous operation. By "semi-continuous" operation is meant the type of operation wherein a continuous mode of simultaneously charging air and o-xylene follows the first batchwise mode of charging all of the components of catalysis and water and heating the same to reaction temperature under reaction pressure, at the completion of charging all the xylene continuing only the charging of air until for all practical purposes the consumption of oxygen ceases and then stopping the air, and discharging all the reaction mixture for processing to recover the o-phthalic acid product. Heating to reaction temperatures of the mixture of water and components of catalysis can be accomplished by heat of reaction released by oxidizing 5 to 25% of the o-xylene added with said mixture.
The simple batchwise and the semi-continuous operations of the above copending patent application both provide liquid reaction products containing 89 to 92 weight percent o-phthalic acid. Said simple batchwise operation produces o-phthalic acid in yields of up to 92 mole percent and consumes 5.5 mole percent of o-xylene by total combustion as determined by measurement of the total oxides of carbon produced. Said semi-continuous operation results in an 85 mole percent yield of o-phthalic acid and a 9.7 mole percent total combustion of o-xylene.
The results of said semi-continuous neat air oxidations of o-xylene demonstrate that operating conditions were found to overcome the yield limitation problem of single step oxidation mentioned in British Pat. No. 856,249. Also, from the results of said semi-continuous neat air oxidation of o-xylene one would assume that its operating conditions were applicable for successful operation of truly continuous operation with perhaps the added disadvantage of a higher (above 9.7) mole percent total combustion of o-xylene.
However, such assumption was found not to be correct. Because of the continuous addition of fresh xylene and components of catalysis with the requisite free water and the continuous removal of part of the reaction mixture, continuous operation produces a reaction mixture always containing unreacted xylene. Such difference is in contrast to the batchwise or semicontinuous operations whose reaction mixtures always have a diminishing o-xylene concentration before the reaction mixture is withdrawn from the site of reaction. Such difference in composition of reaction mixture is material because it gives rise to a problem interfering with successful continuous one step oxidation not found during the development of the successful one-step operation by batchwise or semi-continuous operation. Manifestation of said problem arising from the presence of unreacted o-xylene begins, we found, when the liquid reaction mixture contains 40 to 41 weight percent o-phthalic acid. Upon reaching such composition there forms a liquid o-xylene phase and a liquid o-phthalic acid phase which contains the components of catalysis and the requisite 2 to 7 weight percent free water. Said two liquid phases, we found, were immiscible even though the reaction mixture was vigorously stirred. Because the components of catalysis are in the liquid o-phthalic acid phase, the catalysis is not effectively available for rapid oxidation of o-xylene and it accumulates as an increasing immiscible liquid phase. Up to reaching the 40 to 41 weight percent o-phthalic acid concentration the oxidation reaction is vigorous but, as the immiscible liquid o-xylene accumulates, the oxidation reaction diminishes in vigor until the rate of oxidation becomes commercially unacceptable. Such vigor diminishing condition is readily observable from the volume ratio of o-xylene to water condensed from the exhausted from the oxidation zone. Such volume ratio is normally in the range of from 0.3:1.0 to 0.5:1.0 but the reaction's diminishing vigor is indicated by change of such ratio to 1:1 and finally to 2:1 for an unacceptable reaction rate.
Even when the o-phthalic acid concentration reached 40-41 weight percent in the batchwise and semi-continuous oxidation there was, it is submitted, sufficient benzoic and o-toluic acid present in the reaction mixture to make miscible the o-xylene and the liquid o-phthalic acid solution of water and catalyst components.
We have, however, discovered a simple solution to said problem of forming two immiscible liquid phases. Said solution comprises the essential feature of the present inventive contribution to the continuous single step oxidation of o-xylene with substantially complete conversion thereof to o-phthalic acid yields of 90 mole percent and with total combustion of o-xylene of less than 9 mole percent.